If a boy calls you "big sister", it usually means that he regards you as his sister or close friend. If you feel comfortable, you can respond to him and tell him that you are glad that he regards you as his sister or friend. For example, you can say, "Thank you for your concern. I am glad that you regard me as your sister or friend. "
If you don't like the name, you can also tell him how you feel. For example, you can say, "I don't like being called' big sister' very much. I hope you can call me by my first name in the future. "
No matter which response you choose, you should express your thoughts in a sincere and respectful way.